While a motor vehicle is running very normally on an ordinary road, it does not require a very high engine output, and is expected only to produce half the engine output that the motor vehicle can produce. Thus, a motor vehicle furnished with a multi-cylinder engine may be designed so that the engine operation for some of its cylinders can be stopped to reduce the output in this running state, whereby the fuel-cost performance can be improved.
Conventionally proposed, therefore, is a variable-cylinder engine which can be operated in either a cylinder-off state, in which the engine operation is stopped for some specific ones of its cylinders, or a cylinder-on state, in which the engine operation is performed for all of the cylinders. When operating the conventional variable-cylinder engine in the cylinder-off state, rocker arms of the specific cylinders are rocked idle to stop the operation of suction and exhaust valves, whereby the operation of valve drive mechanisms associated with these cylinders is stopped. Thus, in the case of a 6-cylinder engine, for example, the engine operation is stopped with respect to, e.g., three cylinders. In changing the operating state between the cylinder-off state and the cylinder-on state, moreover, the valve drive mechanisms for each cylinder and a fuel supply system (e.g., fuel injection system) are controlled so that they are operated in associated with each other, thereby ensuring normal combustion in the cylinders.
However, the conventional variable-cylinder engine control method is subject to a drawback such that the engine output greatly fluctuates, thereby producing a shock, during the change between the cylinder-off state and the cylinder-on state, or a spark plug smolders.